Proper metering of fuel to a gas turbine engine during start-up, i.e., from ignition of fuel in combustion chambers to the time the turbine spool achieves normal idle rotational speed, is a difficult and demanding task. If insufficient fuel is metered and burned, necessary torque will not be applied to the turbine spool for achieving normal idle rotational speeds. In fact, fuel combustion can become so insufficient that combustion itself cannot be sustained and flameout will occur resulting in unburned fuel flowing on previously heated gas turbine engine components causing vaporization and a risk of explosive reignition. The opposite situation results with excess metered fuel which raises temperatures of gas turbine engine components too rapidly and to excessive levels. Both a rate of temperature increase and excessive quantities of heat cause exorbitant thermal stress reducing gas turbine engine lifetime and possibly risking non-predicted component failure to include fracture. Further, excessive heat and rate of temperature increase can stall gas turbine engine compressors.
Depending on turbine spool rotational speeds, the acceptable range in amounts of metered fuel available during gas turbine engine start-up can be quite limited. The type of fuel, ambient temperature and ambient pressure are among many variables which determine the amount of metered fuel required for efficient and reliable gas turbine engine start-up with minimized thermal stress. Therefore, prescheduling metering of fuel for gas turbine engine start-up without dynamic adjustment for unpredictable influencing variables cannot assure reliable start-ups with minimized thermal stress.
A prior known strategy for scheduling fuel metering is to begin supplying the maximum potentially necessary quantity of fuel to combustion chambers, and then monitor differences between measured exhaust gas temperature (EGT) and predetermined EGT limits so that the amount of metered fuel can be reduced as measured EGT reaches the predetermined EGT limits. This gas turbine engine start-up strategy, known as temperature topping, risks excessive rates of rise in EGT and greater thermal stresses on gas turbine engine components than are absolutely necessary for assuring reliable start-ups.
An example of a rich scheduled fuel supply strategy is provided in U.S. Pat. No. 4,350,008 issued to Zickwolf where scheduling increasing rates of fuel flow during gas turbine engine start-up is described as a function of rotor speed and ambient temperature. A gas turbine engine is cranked until sufficient air flow is available for light-off, at which time fuel is provided at a fixed flow rate with an ignitor in operation to initiate combustion. When ignition occurs, the temperature rise that occurs in the burner is detected by a temperature sensor and fuel flow is increased in accordance with a rate of change schedule that is a function of rotor speed and ambient temperature. A temperature limiting control compares measured turbine exhaust temperature with a reference derived from both rotor speed and ambient temperature. If the measured turbine exhaust temperature exceeds the compared reference value then the rate of fuel change schedule is overridden to reduce fuel flow when overtemperature occurs. Essentially, a rich supply of fuel is provided until a reference temperature is exceeded.
Another fuel control system for use during gas turbine engine start-up is described in U.S. Pat. No. 4,281,509 issued to LaGrone. A fuel control system has a speed governor responsive to engine speed so that fuel flow is variably delivered to the engine with a governor controlled feedback loop arrangement maintaining engine speed. To accelerate the engine along a required-to-run line, the governor speed set point is changed gradually at a rate which is somewhat slower than the corresponding acceleration capability of the engine when accelerating along its required-to-run line. Such scheduling of the governor set speed point is accomplished by a timing mechanism which is responsive to elapsed time for the engine start-up period. Thus, adjustment of the timing mechanism extends the time for gas turbine engine acceleration. Such time extension results in permitting mechanical resonances at intermediate start-up speeds to build up which unavoidably reduce gas turbine engine lifetime. This situation is unavoidable because in selecting a fixed elapsed start-up time, the highest acceleration rate must be lower than optimum acceleration rates to avoid having excess fuel provided and thereby cause compressor instability and possibly even compressor stall.
U.S. Pat. No. 4,274,255 issued to Pollak, describes another closed loop fuel control system for starting gas turbine engines. Fuel flow to a gas turbine engine is adjusted as a function of difference between predetermined torque and measured torque so as to control fuel metering with a predetermined start-up schedule. The predetermined schedule, here torque, must be set low enough to assure the gas turbine engine can produce the scheduled start-up value during the most adverse conditions, which necessarily extends time for start-up.